Prepare the glass: If using coconut for the rim, wet the rim of the glass with a lime wedge and dip it in shredded coconut.
Mix the base: In a cocktail shaker or jar, combine coconut milk, fresh lime juice, and simple syrup.
Shake well: Add a few ice cubes to the shaker and shake vigorously for about 15 seconds until well chilled.
Pour and finish: Fill a tall glass with ice cubes. Strain the mixture into the glass, filling it about two-thirds full.
Add bubbles: Top off with club soda or sparkling water and stir gently.
Garnish: Add a lime wheel and mint leaves if desired.
Serve immediately while cold and bubbly.